Tunable bandpass filter with wide out-of-band rejection

This paper presents tunable microstrip bandpass filters. These filters are based on Stub-Loaded Resonators. By properly loading these resonators by variable capacitance (such as varactor diode), the center frequency can be easily controlled. Thanks to the chosen topology, the loading capacitors have no impact in the out-of-band rejection. A complete resonant mode analysis is investigated. The concept is demonstrated by considering a third-order parallel-coupled bandpass filter. A tunable center frequency of +/- 7.6% is obtained.